The present invention relates to an image forming apparatus using exposing means in which an organic electroluminescence element is used as a light source.
An electroluminescence element is a luminescence device utilizing electroluminescence of a solid fluorescent substance. Currently, an inorganic electroluminescence element using an inorganic species material as a luminescence substance is reduced into practice and application and development thereof to a back light, a flat display or the like of a liquid crystal display is partially achieved.
However, according to an inorganic electroluminescence element, voltage necessary for being luminescent is as high as 100V or higher, blue color luminescence is difficult. Therefore, it is difficult to form full color display containing three original colors of RGB.
Further, according to an inorganic electroluminescence element, a refractive index of a material used as a luminescence substance is very large. Therefore, the material undergoes intensive influence of total reflection at an interface or the like, an efficiency of taking out light into air with regard to actual luminescence is as low as about 10 through 20% and high efficiency formation is difficult.
Meanwhile, researches on an electroluminescence element using an organic material have long attracted attention and various investigations have been carried out, however, since a luminescence efficiency is very poor, the researches have not progressed to a full scale research on reduction to practice.
However, in 1987, there has been proposed an organic electroluminescence element having a laminated layer structure of a function separating type dividing an organic material into two layers of a hole transporting layer and a luminescent layer by C. W. Tong et al. of Kodak Company. It has been found that a high luminescent brightness equal to or higher than 100 cd/m2 has been achieved regardless of low voltage equal to or lower than 10V [refer to C. W. Tang and S. A. Vanslyke; Appl. Phys. Lett. 51(1987)913 etc.].
Thereafter, an organic electroluminescence element has started to suddenly attract attention, currently, researches on an organic electroluminescence element having a similar laminated layer structure of a function separating type are intensively carried out. Investigations are carried out sufficiently particularly on high efficiency formation/long service life formation which is indispensable for reducing an organic electroluminescence element into practice and in recent years, a display or the like using an organic electroluminescence element is realized.
Here, an image forming apparatus by an electrophotography technology is provided with exposing means for irradiating exposure light in accordance with image data to a photosensitive member charged uniformly at predetermined potential and writing an electrostatic latent image on the photosensitive member. Further, a primary exposure system of in the exposing means is constituted by a laser beam system or an LED array system.
When the exposure system is constituted by laser beam, a space occupied by an optical part of a polygon mirror, lens of the like is large and it is difficult to downsize the apparatus. Further, in the case of the LED array, since the board is expensive, the cost of the apparatus is difficult to reduce.
Further, when the above-described organic electroluminescence element is used for the light source, the problems can be resolved.
Further, there is an element structure of an organic electroluminescence element disclosed in JP-A-10-1664 or Japanese Patent Publication No. 2001-63136.
Here, an image forming apparatus includes a heat source at an inner portion thereof such as a fixer for fixing a toner image transcribed on a record medium.
Further, an organic electroluminescence element is liable to undergo influence of heat and as shown by FIG. 9, with rise in environmental temperature, a time period until brightness half-life is shortened. This signifies that with rise in environmental temperature, element life of an organic electroluminescence element is acceleratingly shortened.
Further, as shown by FIG. 10, relative brightness is significantly changed in accordance with the change in environmental temperature. This signifies that darkness of a developed image is changed by the change in the environmental temperature.